lab_append Appendix C Measurement of liquids Accurate, meaningful data for the laboratory exercises in this manual require that you develop both the knowledge and skill to measure and transfer very specific quantities of liquid with both accuracy and precision. Although the use of instruments for the measurement of volume is widespread, few students have been exposed to the correct techniques for volume measurement. Even though many students may say "I know how to pipette", few can distinguish between a "to deliver" and a "to contain" pipette, determine the appropriate type of pipette for a particular job, or properly use a pipette pump. Instruction in the use of volumetric glassware may seem rudimentary, but their proper use really requires considerable sophistication, and your success in gathering data during laboratory experiments is dependent on your competence with a pipette. Proper measurement of liquids requires choosing the proper glassware and using it correctly. Laboratory glassware The most commonly used types of laboratory glassware used in the measurement of liquid volumes are graduated cylinders, volumetric flasks, pipettes and burettes. However, there is a difference between graduated and volumetric glassware. Graduated beakers, flasks, cylinders, and pipettes have a series of marks indicating different volumes.

The volumes indicated are generally only approximations, sometimes close approximations, but approximations nevertheless. Each graduate (mark) is not calibrated separately, rather, one volume is calibrated and then a standard scale is applied to the glassware using the calibration mark as the starting point. Thus, on a 100 ml graduated cylinder, only the hundred ml volume may be calibrated and then a scale with standard number of equidistant divisions is applied below the calibrated volume. Clearly, this assumes an absolutely true (straight) tube with no variations in diameter. Any variation in diameter between milliliters 50 and 60 would mean that between divisions 52-53 there may actually be more or less than 1 ml, even if the total volume of the cylinder is exactly 100 ml. Inexpensive (cheap) graduated glassware is often not even calibrated at one reference point, but rather just has a scale of graduated values painted on it. The variations due to tube diameter variations, etc. are merely accepted as being within a specified range (100 ml plus or minus 1 ml, etc.). This glassware is never used where accurate measurement is essential.

On the other hand, each volume indicated on volumetric glassware has been carefully calibrated. Thus, a 100 ml volumetric flask has had exactly 100 ml of water or mercury placed in it, then the volume mark scribed on it. As with other measurement equipment, it is extremely important to know which type of glassware should be used for each specified task. If you need about 100 ml of water, a graduated cylinder will do quite nicely and is relatively fast and easy to use. However, if you need exactly 100 ml to make up a "standard" solution, only a volumetric flask would be appropriate. The types of glassware most commonly used in liquid measurement include the following: Graduated cylinders used to measure and transfer various volumes of liquids. The smaller the cylinder diameter, the more accurate the volume measured. Accuracy of graduated cylinders range from 0.5% to 1% of the total volume. Graduated flasks and beakers used to obtain an estimate of the volume of liquid in the container. Graduated flasks and beakers may be off by as much as 10% of the total volume. They are for gross estimates only.

Volumetric flasks used to obtain precise volume measurements. A high quality volumetric flask can have an accuracy of plus or minus 0.05%. They are normally used in the preparation of solutions and also for the precise dilution of solutions (i.e. diluting a solution 1:100 would require the transfer of 1 ml of original solution to a 100 ml volumetric flask, and then filling the flask to its 100 ml calibration mark with solvent). Graduated pipettes pipettes are used for the transfer of solutions and solvents from one vessel to another. As with other graduated glassware, they have an approximate scale consisting of a series of equivolume units that subdivide the maximum volume of the pipette. Their accuracy is similar to that of a graduated cylinder, 0.5 to 1% of the total volume . Again, the smaller the diameter, the more accurately it can be subdivided. Although not extremely accurate, a whole series of approximate volumes can be quickly and easily transferred.

For instance, you could rapidly transfer 0.1 ml of solvent to each of 10 test tubes using a 1.0 ml pipette graduated in 0.1 ml subdivisions by merely filling to the maximum (1.0 ml) line, and then releasing 0.1 ml into each test tube Volumetric pipettes carefully calibrated pipettes used to transfer a single volume of liquid. The liquid is drawn into the pipette until the miniscus is exactly even with the calibration mark. Release of the liquid from this point delivers a single accurate volume of liquid. Accuracy of volumetric pipettes can exceed 0.05% of their total volume Burettes used to deliver approximate volumes of liquid. Because there is a stopcock at the bottom and a tapered, small diameter tip, liquids can be released in very small, accurately measured amounts. These are especially useful for titration. The accuracy of burettes is similar to that of a high quality graduated pipette (0.5%) Syringes used to transfer liquids, particularly if these need to be delivered by injection (penetration of the delivery tip into the organism or through a rubber septum).

Most syringes are graduated, and thus deliver only approximations, even though they are close approximations. There are also glass volumetric syringes whose internal diameters are ground to have extremely small variations and whose plungers are mechanically driven. Because these can deliver extremely accurate volumes of liquids and the rate of delivery can be precisely controlled by a delivery (plunger depressing) screw, they can be substituted for burettes, especially when delivering small total volumes.< Measuring Volume Once you have chosen the proper piece of glassware for the job, there are several important points to be kept in mind when measuring liquid volumes. One point that may seem obvious is to be sure all glassware is clean and dry before beginning a volume measurement. A second point is to be certain to read the volume properly. As the liquids that you will commonly use have a rather high surface tension, they will tend to rise higher along the sides of a container and be lowest in the center. The lowest point in the center is termed the miniscus, and it is with respect to this point that all liquid measurements are made.

Thus a correct volume would be obtained by adding solvent until the miniscus is exactly at the level of the calibration mark (usually a ring scribed in the neck of the flask), not when the highest rim of solvent reaches the calibration mark. Finally, to be redundant, it is extremely important that the proper piece of glassware is used to perform the desired measurement. Pipetting There are a few things that you should know before you start pipetting. On each pipette is given its total volume for measurement, the size of its smallest division, the temperature at which it was calibrated (as liquids change volumes with changes in temperature), and the letters T.D. (to deliver), T.C. (to contain), an upper and sometimes a lower calibration line. Selection Selecting the proper pipette is half the battle in accurate pipetting. A few basic rules will help you to reach for the most appropriate one. Always use the smallest pipette that will contain all of your sample. The smaller the pipette more accurate it will be. Do not, however, use repetitive deliveries from a too small pipette. That is to deliver 3.2 ml use a 5ml pipette not a 10 ml or four deliveries with a 1 ml.

The size information is critical for selection of the proper pippette, while the TD or TC designation and the calibration lines will help you select the proper delivery technique. Filling There are two basic ways to draw liquid into the pipette. One is "by mouth" or using the mouth for suction, and the other is using a "pipette pump". Mouth suction is tempting. It is quick and relatively easy to control and you always have one with you. However, for the pipetting of noxious, poisonous or just plain disgusting liquids it is at best a risky business. A mouthful of concentrated acid, rat urine, or even frog spit, is not recommended. Therefore, for any solution or solvents, the use of a pipette pump is Mandatory! Pipette pumps come in a variety of forms. One is merely a rubber ball with three (intake, exhaust, and suction) check valves. A more contemporary form consists of a cylinder and piston with a thumb wheel control over the position of the piston. The pipette is fitted into the pipette pump with a twisting motion and the liquid is aspirated in to the pipette (either by releasing the rubber squeeze bulb or by pulling back on the piston).

Do not use pipettes with rough or chipped ends in a pipette pump or you will damage the rubber seal. If you cannot get a seal easily, check both the end of the pipette and the rubber seal to be sure that both are smooth. Do not force the pipette or you may break it. If the syringe pump is equipped with control valves, it can be filled to the upper calibration mark with the pump. If the pump is not equipped with control valves, it must be over filled so that you have time to place your finger over the pipette top before the liquid drains past the upper calibration mark. Delivery If your pipette pump is equipped with a delivery valve, you can deliver your measured volume by opening the valve and allowing air into the top of the pipette. The flow of liquid from pipettes equipped with pumps without valves must be controlled with your forefinger: Overfill the pipette, remove the pump, and put your finger over the end of the pipette. You can not accurately control the flow of liquid from a pipette with your thumb. Never use your thumb to control release of liquid from a pipette. When using pipette with both upper and lower calibration marks, the volume delivered is controlled by the user.

If the miniscus drops below the lower calibration line more volume has been released than is calibrated, and the exact volume released is unknown. Clearly, when using these pipette, the miniscus must not go below the lower calibration line. If the pipette is labeled T.D. and has no lower calibration mark, its volume will be delivered when the pipette has completely drained and the tip has been touched to the side of the receiving vessel. If the pipette is labeled T.D., and has a lower calibration mark its volume will be delivered when the miniscus reaches the lower calibration mark. If your pipette is labeled T.C., its volume will be delivered when the entire contents of the pipette has been blown out. Obviously it is critical to know which type of pipette you are using as each type requires a different technique. You should be extremely careful not to get liquids up into the pipette pumps; however, they can be washed out if this happens and, if permanently damaged they can be discarded, something that you can't do with your mouth. Hints to better pipetting Select a proper pipette. Always read the pipette. Is it a T.D or a T.C?

Does it have both upper and lower calibration marks or upper marks only and graduations into the tip? Each uses a different delivery technique so it is critical to know. Whenever possible have your sample as close to the calibration temperature as possible. Always touch the tip of a pipette to the receiving vessel (not the liquid in it) at the completion of delivery. The measurement of very small volumes Contemporary biological laboratory practices have in many cases been scaled down to minimum sizes. This miniaturization allows the conservation of expensive reagents and reduces the amount of potentially hazardous wastes generated. this miniaturization, however, require the transfer of very small volume of liquids (0.01 to 1000ul). Volume Setting The volume indicator consists of three number dials and is read from top to bottom. The three digits indicate the volume selected and are colored black and/or red. The black digits on P-10, and P-20 pipettes show microliters; the red digits show tenths and hundredths of microliters. P-100 pipettes have all-black dials. For the P-1000 pipettes, the digits in red represent milliliters and the digits in black microliters.

For each model, the smallest incremental setting can be set between the digits by referencing the vernier marks. Operation To set a volume: hold the Pipetman in one hand and turn the volume adjustment knob with the other hand until the correct volume shows on the digital indicator. The friction ring "locks" the volume. Turn the knob 1/3 revolution above the desired setting then slowly down to the setting, to prevent mechanical backlash from affecting accuracy. If you pass the desired setting, turn the dial to 1/3 revolution above the desired setting and reset the volume. Attach a new disposable tip to the pipette shaft. Press firmly to ensure a positive airtight seal. Depress the plunger to the first stop. This part of the stroke is the calibrated volume displayed on the digital volume indicator. Holding the Pipetman vertically, immerse the disposable tip into the sample liquid to the proper immersion depth. Allow the pushbutton to return slowly to the UP position. Never let it snap up! Wait a few seconds to ensure that the full volume of sample is drawn into the tip. Withdraw the tip from the sample liquid.

Should any liquid remain on the outside of the tip, wipe it carefully with a lint- free tissue, taking care not to touch the tip orifice. Pipetting Guidelines and Precautions Consistency in all aspects of pipetting procedure will significantly contribute to optimum reproducibility. Attention should be given to: 1. Consistent speed and smoothness when you press and release the pushbutton. 2. Consistent pushbutton pressure at the First stop. 3. Consistent immersion depth. 4. Minimal angle (from vertical). If an air bubble is noted within the tip during intake, dispense the sample to the original vessel, check the tip immersion depth, and pipette more slowly. If an air bubble appears a second time, discard the tip and use a new one. Prevent liquids from being drawn into the Pipetman shaft (with possible resultant damage) by observing the following precautions: 1. Pipette slowly, holding Pipetman no more than 20o from vertical. 2. Never invert or lay Pipetman on its side with a tip installed. There is a great a chance that liquid in the tip will run in to the pipettet and damage it permanently. 3.

For the Model P-5000 Pipetman, use the special safety filters supplied. Proper tips The requirments for disposible pipette tips are demanding. The proper function of the pipettor is dependant on matching the tip exactly to the instrument. Disposable pipette tips are usually color coded to make this task easier. As we use tips from many different suppliers the color code will be provided at the time of the laboratory. Dilution and Concentration It is frequently necessary, for many reasons, to dilute biological samples, and one must know the concentration (at least the relative concentration) of the diluted sample. The simplest example is a 1:2 dilution; the 1:2 designation means 1 part stock (the thing you want to dilute) to 2 parts total. If you wanted to make 10 ml of a 1:2 dilution, you would add 5 ml of the stock to 5 ml of diluent (the thing you dilute with - usually water, buffer or medium).

It makes intuitive sense, in this simple example, that the concentration in the diluted sample is one half that in the stock; if the stock is 10 mM, the diluted sample is 5 mM. In may instances, the concentration of the diluted sample will not be so intuitively obvious, and you will need to calculate it. A valuable relationship to know is: concentration(stock) x volume(stock) = concentration(diluted) Suppose you made a 3:5 dilution of a 45 mM solution: 45 mM (3 parts) = X mM (5 parts). X = 27 mM. When making a 1:10, 1:100, 1:1000 etc. dilution, it is more conve nient to use scientific notation to indicate the dilution factor. You can dilute something 10-fold (a 1:10, or 10-1 dilution) by mixing one volume of the solution to be diluted with nine volumes of the appropriate solvent, e.g., 0.5 ml of yeast stock plus 4.5 ml medium, or 0.1 ml of yeast stock plus 0.9 ml medium.

You could get a 100-fold dilution (1:100, or 10-2) by mixing 1 volume of yeast stock with 99 volumes of medium, e.g., 0.05 ml into 4.95 ml. For many applications, the concentration you need is close, within a couple of orders of magnitude, to that of the stock. In such cases, a single dilution will bring your sample into the needed concentration range. But other samples are much too concentrated, and you'd need to dilute them drastically. For example, what do you do if you want to dilute a yeast suspension that has 109 cells/ml to make one that has 103 cells per ml? Volumes smaller than about 0.05 ml cannot be pipeted accurately without special equipment, so if you were to do it in a single dilution, you would need to add the 0.05 ml of yeast stock to 5x104 ml or 50 l of sterile water! That would be impractical.

If you want to dilute a solution by more than about 10-2 without using huge liquid volumes, you must employ a technique known as serial dilution. That is, you must dilute by a factor of ten or a hundred, and then take this new solution and dilute it again (Figure E-1). Figure E-1. Using serial dilution to make a 10-5 dilution of a yeast stock. A 0.5 ml aliquot from the stock is transferred to the second tube, and the contents are thoroughly mixed. Thus, the second tube represents a 10-1 (10-fold) dilution of the stock. Then, a 0.05 ml aliquot from the second tube is added to the third tube. After mixing, the third tube contains a 10-2 dilution of the contents of the second tube, which is equivalent to a (10-1) Ñ (10-2) = 10-3 dilution of the original stock. Another 10-2 dilution brings the total dilution to 10-5 in the last tube.

Note that the 10-1 dilution and the two 10-2 dilutions can be done in any order with the same result. Walter I. Hatch Measuring Volumes - WikiLectures Measuring Volumes Thank you for your review (★) Volumetric Vessels and other Tools in the Chemical Laboratory[ edit | edit source ] Various laboratory vessels and devices are used for preparation of solutions and manipulation with liquids. They differ in the intended use and in accuracy. Precise volume must always be measured at constant temperature because, due to thermal expansion, changes in temeprature are followed by changes in volume. For example water at 10 °C has much smaller volume than at 80 °C. Volumetric vessels are usually calibrated for 20 °C (25 °C in the USA). Calibration temperature is written at every volumetric vessel. Substantial error may result e.g. from change in temperature during dilution of an acid. A lot of heat is released during dilution and temperature increases. When the volume of acid is measured immediately, the real volume after cooling-down will be smaller.

The requirement for constant temperature must therefore be observed asses Devices for measuring volume of liquids Device * not suitable for measuring volumes, only approximate volume indicated Beakers[ edit | edit source ] Beaker Beakers are used for approximate determination of volume of liquids therefor are not necessarily classified as measuring tools. They are used especially for dissolving compounds, diluting liquids, heating and other operations. Beakers are unique for there 'beak' that is used to pour liquids. Because of low accuracy, they are frequently not classified as volumetric devices. Volumetric Flasks and Cylinders[ edit | edit source ] Volumetric cylinder and volumetric flask Volumetric cylinders and volumetric flasks are used to measure volume of liquids contained in them. They are calibrated for volume included in them - this is indicated by the marking "IN". The liquid has accurate volume when it reaches the corresponding marking on the scale. Volume is usually indicated in mL. During measurement of the volume the vessel must be placed on an even, horizontal support. The required volume is measured when the lower edge of meniscus of the liquid just touches the marking on the vessel. Cylinders are less accurate, volumetric flasks are used for preparation of solutions of exact concentration.

Cylinders are used solely for measuring volume, they are not employed for dissolution, dilution or mixing. Burettes, Pipettes, Dispensers and Syringes[ edit | edit source ] Burettes, pipettes, dispensers and syringes measure volume of a liquid delivered to another vessel. Pipettes and burettes are usually calibrated for outpouring. It is indicated by letters "EX" (from exclude). The exact volume is measured when the liquid is kept to flow out from a certain marking on the scale. Some liquid may remain in a pipette after outpouring; it should never ever ever be blown out, its volume was taken ito account during calibration. Burettes. Burettes Burettes are used for titrations or whenever the same liquid is repetitively measured. Burette is a glass or plastic tube with calibrated scale closed with a stopcock. Burette is attached to a stand in a vertical position. With a closed vent, it is filled with the measured liquid. Some liquid is then outpoured so that the meniscus touches marking of the scale. Now, burette is ready for titration.

The liquid is outpoured with the stopcock and the volume is read on the scale. The most important moment is reading the volume; with burette, the volume is always read twice: first to read the starting position of the meniscus and the second time to read the end position. As the difference in volume is calculated, it is not important how exactly the volume is read, it must however be read in the same way every time. Automatic burettes are employed in routine laboratories. Glass pipettes. Glass pipettes Today, glass pipettes are rarely used in contemporary routine laboratories; glass pipettes were replaced by semiautomatic dispensers. The volume of glass pipettes may vary from 1 to 100 mL. Glass pipettes can be non-scaled for measuring of a certain volume or scaled with grades by 1 mL and tenth of milliliter. Numbering of the scale can go from the tip up or in the opposite direction. For security reasons, liquid should never be aspired by mouth. Various adapters and pistons are used instead. Adapters for glass pipette When aspiring the solution, the pipette should never touch the bottom of the vessel.

Before measuring a sample the pipette should be filled with the solution first and the liquid is then thrown away. Then the exact volume is measured and delivered for further processing. The measured solution must never get into the adapter. Automatic pipettes Pipettors (automatic pipettes, micropipettes, microdispensers). Another possibility of measuring small volumes is use of micropipettors. They are always calibrated for outpouring. Microsyringes. Microsyringes serve for dispensing small volumes (0.1–1000 μL) of liquids. They consist of a needle attached to a scaled glass cylinder with a piston. Different types vary in the diameter of needles and pistons. Microsyringe edit | edit source ] Micropipettes without disposable tip According to the way of operation, manual and electronic pipettors can be distinguished. In manual pipettors, the piston is moved by a thumb using an operation knob. Accuracy and precision of pipetting depend on expertise of the operator. In case of electronic pipettors, the piston is moved by a small electric motor. Different speeds of aspiring and expressing of the liquid can be selected according to properties of the pipetted solution.

According to the principle of operation, pipettors can be divided into two big groups: „Air displacement“ pipettors Principle of so called air cushion is employed in this type of pipettes. A certain volume of air is between the piston and measured liquid. The volume of solution and of the air aspired or expressed with the piston may differ. It depends on density and viscosity of the liquid, wettability of the surface of the tip with pipetted liquid, temperature, athmospheric pressure and other influences. Thus, every pipettor must be calibrated on a regular base. Air displacement pipettors can be designed as a single channel (for pipetting of a single volume at a time) or as a multi channel (most frequently with eight or twelve channels). Multi-channel pipettors are intended for simultaneous pipetting of the same volume of liquid to several wells of a microtitration plate . Every channel has its own piston; therefore, it is not necessary to use all 8 or 12 channels (i.e. less tips can be attached to the pipette). Micropipettors are constructed for one fixed volume or it is possible to set the volume.

Volumes can be set discretely (changing of plug-in modules) or continuously in a certain range (e.g. 10–100 μL) using an adjustable screw or knob. At the lower end of a micropipette there is a removable tip. At the other end, there is the knob operating the piston inserted into the cylinder of the pipettor. „Positive displacement“ pipettors Liquid is aspired into the tip without any air cushion – the piston is in contact with measured liquid. The liquid is then delivered at once or in several steps (in case of so called "steppers"). This type of pipettors is suitable for very viscous or voilatile liquids and for repetitive pipetting. Operating a Pipettor[ See also comparation of actual and checked version . Forward pipetting This is the most frequently used technique. In forward pipetting, an exactly set volume of liquid is aspired to the tip and then it is delivered to a new vessel. This technique is recommended for pipetting of diluted aqueous solutions, bufers, diluted acids and bases. Forward pipetting Procedure: Attach a tip to the pipettor. Press the button to the first stop (it is necessary to overcome a slight resistance).

Dip the tip approx. 2-3 mm under the solution level. Release the button slowly; the solution is aspired to the tip. Slow aspiration prevents tubulences that would lead to formation of aerosol and bubbles of gas dissolved in the solution. The optimal speed depends on properties of the liquid (density, tension of vapors and viscosity). Pay attention not to aspire air bubbles (e.g. when the piston is released to quickly or if the tip is not attached properly). Higher accuracy can be reached if the thumb is completely removed from the button after it has reached the released position. Take the tip off the liquid slowly. Fast removal may lead to loss of a portion of the content. Before removing the tip, wait several seconds especially when working with larger volumes (500–5000 μL). When necessary, wipe droplets from the external surface of the tip with tissue cloth. Never touch the orifice of the tip, the cloth would absorb a portion of the transfered volume. During delivery of the liquid, the tip should touch a wall of the vessel at an angle (10–45º). It should be just above the level of any liquid that already is in the vessel. Push the button to the first stop.

Wait for approx. 1 second and push the button quickly to the second stop (a stronger resistance can be felt). No droplets should remain in the tip and no liquid should be splattered on the walls. Hold the button in the second position and remove the tip, still touching the wall of the vessel. Now you can release the button. Forward pipetting brings a small error in the delivered volume as a thin film of pipetted solution remains on the inner walls of the tip. In other words, according to the described procedure a slightly smaller volume than required is delivered. This error depends on properties of measured liquid and of the tip. The error can be avoided if the inner wall of the tip is pre-rinsed with the measured liquid. Practically, liquid is first aspired to the tip. Then it is not delivered to a new vessel but returned back to the stock vessel. Now, a very thin film of pipetted liquid covers the inner wall of the tip (it is usually invisible). Pipetting according to instructions given above follows (without changing the tip). As the volume of liquid remaining in the tip is practically constant, exactly the set volume is measured.

Reverse technique In reverse technique, a larger volume of solution is aspired to the tip. Then, an exaxt volume is delivered (and some liquid remains in the tip). This technique is suitable for very viscous or volatile liquids, biologic fluids, foaming solutions and for measuring very small volumes. Reverse pipetting Procedure: Push the button to the second stop. Dip the tip 2–5 mm bellow the level of the solution. Slowly aspire the solution to the tip. Remove the tip slowly from the solution. If necessary, wipe droplets from the external surface of the tip. Deliver the liquid to a new vessel, pushing the button to the first stop only. The tip should touch the wall of the vessel similarly to the forward technique. Hold the button in the first position and remove the tip from the vessel. The liquid remaining in the tip can be returned to the stock vessel or thrown away. Repetitive pipetting Repetitive pipetting This technique is used when the same volume of the same liquid is to be measured to several test-tubes or to many wells of a microtitration plate. It is similar to reverse pipetting where steps 2 to 4 are repeated several times.

Pipetting of heterogenous samples This technique is suitable for heterogenous samples like whole blood. Pre-rinsing of the tip before pipetting is not easy in this case. Pipetting heterogenous samples Procedure: Push the button to the first stop and dip the tip approx. 2–5 mm beneth the solution level. Slowly release the button. The sample is aspired to the tip. Remove slowly the tip from the solution and wipe droplets on the outer wall of the tip. Dip the tip into the target solution. Push the button to the first stop and then slowly release back. The solution will be aspired. Do not remove the tip from the solution and repeat this step until the inner wall of the tip is clean. Touching the wall of the vessel, place the tip above the level of the solution and push the button to the second stop. Hold the button at the second stop and remove the tip from the vessel. Links[ Laboratory volumetric glassware used in titration - burette, pipette, ASTM E287-02 standard specification Titration » Burette, pipette, flask - volumetric glassware During titration experiments you will be using several types of volumetric glass.

They all are designed to help measure volume of a liquid. Some types of the volumetric glass can be used only to measure predefined volume of solution. These are volumetric flasks and single volume pipettes. They are characterised by a a high accuracy and repeatability of measurements. Flasks are designed to contain (TC, sometimes marked as IN) known volume of the solution, while pipettes are generally designed to deliver (TD, sometimes marked as EX) known volume (although in some rare cases they can be designed to contain). This is an important distinction - when you empty pipette you deliver exactly required volume and you dont have to worry about the solution that is left on the pipette walls and in pipette tip. At the same time you will never know how much solution was in the pipette. On the contrary, volumetric flask is known to contain required volume, but if you will pour the solution to some other flask you will never know how much of the solution was transferred. Both kinds of glass were designed this way as they serve different purposes. Volumetric flask is used to dilute original sample to known volume, so it is paramount that it contains exact volume.

Pipette is used to transfer the solution, so it is important that it delivers known volume. Note, that volumetric pipettes are designed in such a way that after a fluid is dispensed, a small drop of liquid will remain in the tip. In general you should not blow this drop out. The correct volume will be dispensed from the pipette if the side of the tip is touched to the inside wall of the flask (or beaker). Third kind of precise volumetric glass is burette. Burette is used to add titrant to the titrated solution and it has a scale on the side, so that you can precisely measure volume of the added solution. Burette is similar to the pipette, as it is designed to measure volume of the delivered liquid, but it can measure any volume of the solution. Two other types of volumetric glass are graduated pipettes and graduated cylinders. These are too designed to deliver requested amount of solution and they have a scale on the side. However, their accuracy is usually much lower than the accuracy of volumetric glass described above. They are used to measure amounts of auxiliary reagents, like buffers.

Usually when measuring volume of the solution, the bottom of the concave meniscus must be precisely on a calibration mark. To make reading of the meniscus position easier we can use piece of paper with a horizontal black stripe, about an inch and half wide. If paper is hold half an inch behind a burette with a stripe about a half an inch below meniscus, solution surface seems to be black and is much easier to see. Reading volume on the graduated pipette (or burette) - 1.4 mL. Meniscus surface is in fact a little bit below the 1.4 mL mark, so you may read it as 1.42 mL, assuming it is about 1/5 of the scale distance. So called Schellbach burettes have additional thin, colored line embedded in the glass. This line, when watched through the meniscus, seems to be hourglass shaped - and you should align the thinnest part of the line with the calibration mark. Reading volume on the Schellbach burette - 42.25 mL (that is 42.2 and half of the mark).

In the case of dark solutions (like permanganate), that won't let you see through, meniscus is invisible, and you should align top of the solution with the calibration mark. For obvious reasons this procedure works only for burettes. Volumetric glassware used in labs can be either A class or B class (or non classified). A class glassware is more accurate. Details are covered in the Standard Specification for Laboratory Glass Graduated Burets (ASTM E287-02) . Note that ASTM standards, while adopted worldwide, may be different from your national standards. Required tolerance for volumetric glassware capacity mL 1 Minimum delivery time for Class A serialized and non-serialized (maximum delivery time 60 sec). 2 Calibrated to deliver. 3 Calibrated to contain. Class B volumetric glassware has ±mL tolerances twice those of Class A glassware. Most popular burettes are 10 mL, 25 mL and 50 mL types. 10 mL burettes are usually graduated each 0.05 mL, while 25 mL and 50 mL burettes are usually graduated each 0.1 mL.

That means that 50 mL burettes have the highest resolution. 0.050 mL out of 50 mL is 0.1%, and that's about maximum precision that we can get from volume measurement when using burette. In turn that's also about the maximum precision of the titration. We will use these numbers - 50 mL burette, 0.050 mL volume, 0.1% accuracy - throughout the site, when discussing different aspects of titration. It can be interesting to check relative accuracies of volumetric glass calculated wih the use of the tolerances data. Maximum relative errors for A class volumetric glassware capacity mL 0.500 0.03 Relative errors for burettes are calculated assuming 80% of the burette volume was used during titration. There is one, obvious conclusion form the table - the lower the volume of glasware, the higher the relative error. Thus, for high precision work we should use glassware of higher volume.

As it often happens, this is not a rule to be followed blindly - in the case of small samples large volumes mean dilution, which in turn may mean problems with the end point detection, or larger distance between end point and equivalence point. Note, that for really precise applications you should calibrate pipette and volumetric flask. This is done by precisely weighing water dispensed from the pipette and weighing empty and full flask. Depending on the glass class difference between nominal and real volume can be neglected or have to be taken into account when calculating titration result. Some examples of the markings on the volumetric glassware follows. Note, that these pictures were taken in the lab in Poland, using glass made according to local standards. These are similar, but different from ASTM E287-02. Markings on the B class volumetric flask. Flask is calibrated to contain (In) 1000±0.80 mL of liquid at 20 °C. According to ASTM E287-02, tolerance of A class 1000 mL volumetric flask should be ±0.300, that means ±0.600 for B class flask (twice A class tolerance).

However, this flask is marked DIN - Deutsches Institut für Normung - and DIN standard is slightly different. Markings on the volumetric cylinder. B to the right means B class, calibrated to contain (In) at 20°C. No idea why this cylinder is calibrated to contain. Markings on the B class 10 mL pipette. Calibrated to deliver (Ex) at 20°C. Once again tolerance slightly differs - according to ASTM E287-02 it should be ±0.04. Markings on the A class single volume pipette. Calibrated to deliver (Ex) 15 mL ±0.03 at 20°C, you should wait for at least 15 seconds, touching side of the pipette tip to the inside of the flask (beaker) till solution is delivered. Page was last modified on June 17 2009, 12:07:29.

Tools Used to Measure the Volume of a Liquid | eHow Tools Used to Measure the Volume of a Liquid  Email Save In the sciences, the “tools” for measuring the volumes of liquids are generally made from glass, plastic or occasionally metal, although they are generically referred to as “glassware.” Scientists, and chemists in particular, have a variety of glassware at their disposal for measuring volumes. The particular piece of glassware chosen in any situation will depend primarily upon two factors: the required volume and the accuracy required for the measurement. Beakers and Flasks Beakers and Erlenmeyer flasks can be used to make coarse measurements of volumes, provided that graduated volume levels are printed on the side of the beaker or flask (not all beakers and flasks have these marks). They are usually accurate to within 5%. Graduated Cylinders Graduated cylinders are transparent cylinders with finely divided markings--otherwise known as graduations--marked on their side. They represent a significant improvement in accuracy over beakers and flasks--generally to within 1%. Thus, a 10 mL graduated cylinder will be accurate to within 0.1 mL.

Graduated cylinders are manufactured in sizes ranging from 5 mL to 2000 mL. As with beakers and flasks, graduated cylinders are available in either glass or plastic; glass is easier to clean, but more fragile and expensive than plastic. Burets To a scientist, a large difference exists between a volume of 25 milliliters (mL) and 25.00 mL. The first quantity only requires a precision of 0.5 mL; that is, the measuring device only needs to be able to measure an actual volume that’s within a few tenths of 1 mL. Measuring 25.00 mL, however, requires a device capable of measuring to within a few one-hundredths of a milliliter. Glassware with such accuracy is categorized as “volumetric” glassware. Burets fall into this category. Burets are also cylindrical pieces of glassware with graduations painted on the side, but they have a valve at the bottom (called a “stopcock”) that allows the liquid to flow out the bottom. They are typically accurate to within 0.01 mL.

Burets are available in sizes from 10 mL to 100 mL, although 50 mL is the most common size. Pipets Pipets are slender tubes, typically 12 to 24 inches long. They may measure a predetermined volume such as 25.00 mL or 10.00 mL. They may also have graduations (these are called “Mohr” pipets) that allow odd and fractional volumes to be delivered. They are generally accurate to within 0.02 mL and are thus classified as volumetric glassware. The liquid is typically drawn into a pipet by a rubber bulb that is squeezed, thus creating a vacuum on the top of the pipet as the bulb expands. The operating principle is much the same as sucking liquid through a straw, but without the hazard of requiring mouth-to-glassware contact, which is strictly prohibited in laboratories. Some pipetes are single-use devices made of disposable plastic. Volumetric Flasks Volumetric flasks are designed to contain a fixed volume, such as 100 mL or 1000 mL.

They are typically accurate to within 0.05% and are used to prepare solutions of known concentration.

Question: What name is given to the graduated glass tube used to measure small precise amounts of liquids during titration in chemical analysis?
Answer: Burette